OCR GCSE Biology A: Material cycling and decomposition in ecosystems (J247)

This guide covers statements B4.1a to B4.1d of the OCR GCSE (9-1) Gateway Science Biology A specification (J247), Version 4.0. They sit in topic B4, Community level systems, within B4.1 Ecosystems. None of these four statements is printed in bold, so none is marked as Higher Tier only: the content is for both tiers. The other statements in B4.1 (levels of organisation, abiotic and biotic factors, feeding relationships and biomass transfer) are not taught here. Related pages: the revision notes and the practice questions. Course hub: OCR GCSE Biology. Printable checklist. Not sure where you stand? Try a free diagnostic.

What you must be able to do

Statement	What the specification asks
B4.1a	Recall that many different materials cycle through the abiotic and biotic components of an ecosystem (for example nitrogen and carbon)
B4.1b	Explain the role of microorganisms in the cycling of materials, including decomposition
B4.1c	Explain the importance of the carbon cycle and the water cycle to living organisms: maintaining habitats, fresh water, the flow of nutrients, and the stages of each cycle
B4.1d	Explain the effect of temperature, water content and oxygen availability on the rate of decomposition, using the terms aerobic and anaerobic

The mathematical learning outcomes for this unit are calculating rate changes in the decay of biological material, calculating a percentage of mass, using fractions and percentages, plotting graphs with suitable scales, and extracting information from charts, graphs and tables. The specification suggests investigating the most favourable conditions for composting, so expect experiment questions too.

B4.1a: Materials cycle

An ecosystem receives a steady flow of energy from the Sun, but its materials are not replaced from outside. The same atoms are used again and again. Materials move between the abiotic components (air, water, rock, soil) and the biotic components (living organisms and their remains).

The specification gives carbon and nitrogen as examples, and the water cycle is also named in B4.1c. The key idea is that an atom leaves the abiotic part of the ecosystem, becomes part of an organism, and later returns.

Material	Abiotic store	Biotic store
Carbon	Carbon dioxide in air and dissolved in water	Carbohydrates, proteins, fats and DNA in organisms
Nitrogen	Nitrogen gas in air; nitrate and ammonium ions in soil	Proteins and DNA in organisms
Water	Oceans, lakes, rivers, water vapour, soil water	Cytoplasm, sap, blood

The specification names the nitrogen cycle only as an example of a cycled material and lists no stages for it. Know that nitrogen in dead organisms and waste returns to the soil because of microorganisms (next section).

B4.1b: The role of microorganisms

Dead plants, dead animals and waste such as faeces contain carbon compounds and mineral ions. If nothing broke them down, the materials would stay locked up and producers would run short of mineral ions. Decomposition (decay) releases them.

The main decomposers are bacteria and fungi. Larger organisms such as earthworms help by breaking material into smaller pieces, which gives microorganisms more surface area, but the chemical breakdown is done by the microorganisms.

How a decomposer works:

1. It secretes enzymes onto the dead material (digestion happens outside the cell).
2. The enzymes break large insoluble molecules (such as proteins, carbohydrates and fats) into small soluble molecules.
3. The microorganism absorbs some of these molecules and uses them for growth and as a respiratory substrate.
4. It respires, releasing carbon dioxide into the air or water.
5. Mineral ions that are not used pass into the soil, where plant roots can absorb them.

The result is that carbon goes back to the air or water as carbon dioxide, and mineral ions go back into the soil. Without decomposers, the cycles stop.

B4.1c: The carbon cycle and the water cycle

The carbon cycle

Carbon dioxide is the only route by which carbon enters most food webs. Here are the stages in order.

Stage	What happens	Carbon moves from… to…
Photosynthesis	Producers use light energy to build glucose from carbon dioxide and water	Air or water to producers
Feeding	Consumers eat producers or other consumers	One organism to the next
Respiration	All living things, including decomposers, release carbon dioxide	Organisms to air or water
Death and waste	Dead organisms and waste become food for decomposers	Organisms to dead material
Decomposition	Microorganisms break down dead material and respire	Dead material to air (as carbon dioxide)
Combustion	Burning wood or fossil fuels releases carbon dioxide	Fuel to air

Over millions of years, some dead material that is not fully decomposed can become fossil fuels, which holds carbon out of the cycle until it is burned.

A simple flow to remember:

CO2 in air --photosynthesis--> producers --feeding--> consumers
   ^                              |                     |
   |-------respiration------------+---------------------+
   |-----decomposers' respiration--- dead material & waste <--- death
   |-----combustion---------------- fuels, wood

Why the carbon cycle matters to living organisms. Carbon is in every organic molecule, so every organism needs a supply. Photosynthesis takes carbon dioxide from the air and respiration and decay return it, so the amount available stays roughly steady and producers can always find it. The cycle also passes carbon, and the energy stored in it, along food chains. This is the flow of nutrients the specification refers to.

The water cycle

Stage	What happens
Evaporation	Liquid water from seas, lakes and soil changes to water vapour, driven by heat from the Sun
Transpiration	Plants lose water vapour from their leaves, which adds to the air
Condensation	Cooling water vapour forms clouds
Precipitation	Rain, snow or hail falls
Run-off and drainage	Water flows over land and through soil into streams, rivers, lakes and the sea
Uptake	Plant roots absorb soil water; animals drink

Why the water cycle matters. Precipitation supplies fresh water to land habitats, rivers and lakes. Without it, land would dry out. Water is the solvent in which mineral ions are carried through soil and into plants, it is a reactant in photosynthesis, and it makes up most of the mass of cells. The supply of water also maintains habitats: ponds, rivers and wetlands exist because water is replaced as fast as it is lost, and a change in rainfall changes which organisms can live in a place.

B4.1d: Rate of decomposition

Decomposers are living organisms using enzymes and respiring, so the same factors that affect enzymes and respiration affect decay.

Factor	Effect on rate	Reason
Temperature	Rate rises as it warms, up to an optimum; then it falls; at very low temperatures decay is very slow	Enzymes and microorganisms work faster in warmth; above the optimum enzymes denature and microorganisms die
Water content	Too dry: slow. A moist mix: fastest. Waterlogged: slow	Microorganisms need water for enzyme action and growth; too much water fills the air spaces and removes oxygen
Oxygen availability	More oxygen means faster decay	Aerobic respiration releases more energy and supports fast-growing microorganisms

Aerobic means with oxygen. Anaerobic means without oxygen. Aerobic decay is faster. In waterlogged soil, deep mud or tightly packed material there is little oxygen, so decay is anaerobic and slow; some microorganisms produce methane in these conditions. In a peat bog the water and acidity keep decay so slow that plant remains build up as peat.

Composting uses this knowledge. A compost heap decays fastest when it is warm, moist but not soggy, and turned so that air reaches the microorganisms.

Investigating decay

A typical investigation: put equal masses of the same dead material (such as leaves or grass cuttings) in mesh bags, keep them under different conditions, and reweigh at intervals.

• Independent variable: the factor you change (temperature, water content or oxygen).
• Dependent variable: loss of mass.
• Control variables: starting mass and type of material, size of pieces, time, and the other two factors.
• Repeat each condition and calculate a mean. Weigh after blotting off surface water, or compare dry masses, so water is not mistaken for decay.

Worked examples

Example 1: rate and percentage loss

Grass cuttings of mass 250 g are left in a mesh bag. After 12 days the mass is 187 g. Calculate the percentage loss and the mean rate of decay.

Loss of mass = 250 - 187 = 63 g
Percentage loss = 63 / 250 x 100 = 25.2 %
Rate = 63 / 12 = 5.25 g per day

The mean rate of decay is 5.25 g per day. The cuttings lost 25.2 % of their mass.

Example 2: interpreting temperature data

Equal 100 g samples of leaf litter were kept in moist, well-aerated conditions for 10 days at different temperatures.

Temperature (°C)	5	15	25	35	45	55
Mass lost (g)	4	14	31	46	22	3

(a) Describe the pattern. Mass lost rises from 4 g at 5 °C to a maximum of 46 g at 35 °C, then falls to 22 g at 45 °C and 3 g at 55 °C.

(b) Compare the rate at 35 °C with the rate at 15 °C. At 35 °C the rate is 46 / 10 = 4.6 g per day. At 15 °C it is 14 / 10 = 1.4 g per day. 4.6 / 1.4 is about 3.3, so decay is about 3.3 times faster at 35 °C.

(c) Explain the fall after 35 °C. Above the optimum the enzymes of the decomposers denature, and many microorganisms are killed, so less material is broken down. The best temperature lies near 35 °C, though the data only show that it is between 25 °C and 45 °C.

To plot this, put temperature on the x-axis (the independent variable) and mass lost on the y-axis. Pick scales so the plotted line spreads across most of the grid, such as 0 to 60 °C and 0 to 50 g, and join the points with a smooth line or straight ruled lines.

Example 3: water content

A gardener starts each of five 400 g compost samples with a different water content and records the mass lost in one week.

Water content (%)	10	30	50	70	90
Mass lost in a week (g)	6	38	72	55	15

At 50 % water, the loss is 72 / 400 x 100 = 18 % of the starting mass. At 70 %, it is 55 / 400 x 100 = 13.75 %. Moving from 50 % to 70 % water reduces the loss by (72 - 55) / 72 x 100 = 23.6 %.

Explanation: very dry samples (10 %) leave microorganisms without water for enzyme action. Very wet samples (90 %) have air spaces filled with water, so oxygen is scarce, aerobic respiration is limited and decay slows. The best value in this set is at 50 %.

Example 4: fractions

A sample of 480 g loses three-eighths of its mass. The fraction remaining is 5/8, so 480 x 5/8 = 300 g remains. As a percentage, 3/8 = 37.5 % lost and 62.5 % remains.

Common errors

• Saying “decomposers eat the dead plant”. They secrete enzymes and absorb the soluble products.
• Writing that plants “use up” carbon dioxide so it runs out. Respiration and decay return it.
• Putting evaporation only at the sea. Plants (transpiration) and soil also return water to the air.
• Saying warmth always speeds decay. Beyond the optimum, enzymes denature.
• Explaining slow decay in a wet heap as “no water”. Too much water displaces oxygen.
• Mixing up aerobic and anaerobic, or forgetting that the terms are about oxygen.
• Leaving out the unit (g per day) or confusing the percentage loss with the rate.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018, published by OCR: statements B4.1a to B4.1d and the associated mathematical learning outcomes BM4.1i to BM4.1v.